High performance field sequential liquid crystal color display systems can develop high resolution multicolored images with strong contrast and good color purity. However, a well-known problem associated with liquid crystal switching devices may cause display systems incorporating them to produce colored images of relatively poor quality. Specifically, the speed of response of a liquid crystal device is known to be relatively slow in switching from the field-aligned ("ON") state to the relaxed ("OFF") state. This causes the information of one color field to be displayed in the color of the immediately preceding color field during the transition from the "ON" state to the "OFF" state of the device.
A solution to this problem is suggested in U.S. Pat. No. 4,295,093 of Middleton who describes a spectrum analyzer which employs a liquid crystal switching device that is split into first and second contiguous regions of liquid crystal material, each of which is controlled separately by a voltage applied by a different output of a drive circuit. Both regions share a common electrode which is held at a fixed reference potential. In Column 4, lines 21-38, the Middleton patent describes with reference to FIG. 2 thereof the construction of the liquid crystal device as follows. "[The device] consists of a film 20 of liquid crystal material of the twisted nematic kind . . . One side of the film 20 is provided with a single transparent electrode 21 which is held at a fixed potential, and the other side is provided with two transparent electrodes 22 and 23 which each cover a respective half of the film, and these are connected to the colour select circuit 15 via conductors 24 and 25."
The above-described liquid crystal device is positioned in front of a cathode ray tube screen that is raster scanned by an electron beam. The split line separating the first and second contiguous regions of the liquid crystal device is oriented substantially parallel to the line scanning direction of the raster scanning pattern. In operation, the first region is scanned in its first color state while the second region of the liquid crystal material is switched from a second color state to a first color state. Upon completion of the scan of the first region, the second region is scanned in its first color state while the first region is switched from the first color state to the second color state.
Alternate switching between color states of the first and second regions of the liquid crystal device allows the previously scanned region to settle to its other color state prior to the return of the electron beam to scan that region of the display.
U.S. Pat. No. 4,328,493 of Shanks et al. describes a color display which employs a liquid crystal device constructed with first and second contiguous regions of liquid crystal material as in the Middleton patent. The Shanks et al. patent describes a timing sequence for signals developed at the outputs of a drive circuit which applies AC pulses of one of two frequencies alternately to the first and second regions of liquid crystal material. Light rays of two colors alternately appearing at the output form a multicolored image in a manner similar to that described in the Middleton patent.
The liquid crystal devices described in the Middleton and Shanks et al. patents suffer from the disadvantage of introducing optical cross talk between the contiguous regions of the liquid crystal device. The optical cross talk is characterized in that the electric field applied to the region being scanned by the electron beam is coupled to the unscanned region and causes a spurious transmission of light from it. The two contiguous regions of liquid crystal material are, therefore, not electrically isolated from and optically independent of each other. The result is a color display system with degraded image quality.
One of the objects of this invention is to provide a high performance field sequential liquid crystal color display system that develops a high resolution multicolored image with strong contrast and good color purity.
Another object of this invention is to provide in a display system a liquid crystal switching device that is divided into cell segments which are driven by electrical signals presented in a predetermined sequence to minimize the amount of optical cross talk among the cell segments.
A further object of this invention is to provide in a display system a method and an apparatus for separately driving the cell segments of a liquid crystal switching device to decrease the time between successive color fields and thereby improve the image color quality of the display system.
Still another object of this invention is to provide such a method and an apparatus for driving a liquid crystal device whose cell segments share a common electrode structure.